Cosc 4750

1. Cosc 4750 File Systems

2. Which would you expect to find in a “filesystem”? • Processes • serial ports • inter-process communication channels • Shared memory segments • applications and files

3. All of them are in a UNIX filesystem • UNIX unifies all objects of the O/S into the filesystem • consistent programming interface, easy access from the shell

4. File system components • There are 4 main components • A namespace • Way of naming things and arranging them in a hierarchy • an API • A set of system calls for navigation and manipulating nodes • Security Model • a scheme for protecting, hiding, and sharing things

5. File system components (2) • An implementation • Code that ties the logical model to an actual disk • Pathnames • the directory structure can be as deep as needed, but no name can be longer and 255 characters • But a full pathname can not be accessed properly when it is longer than 4,095 characters

6. File system components (3) • Special characters such as the space, *, ?, and others must either be backslashed or the path must be in double quotes • cat My\ File.txt • cat “My File.txt” • NEVER put a dash as the first character. The – is taken as parameter to the program, not as the filename.

7. Mount/unmouting file systems • You can use the mount command and umount command • mount the zip drive into the filesystem • mount /dev/hdc4 /media/zip • Assuming /dev/hdc4 is the device for the zip drive. • mount /media/zip • This assumes zip drive is listed in the /etc/fstab file. • umount /media/zip • unmounts the zip drive from the filesystem

8. Problems unmounting • Unix will not allow you umount a file system that is in use. • umount /home • umount: /home: device is busy • A file is open or a user’s current working directory is on the file system • To find them, you can use the fuser –mv <file system> command • fuser –mv /home • Show all processes user the /home file system • You can also use lsof command on RedHat/Fedora and SuSE installations.

9. File Structure • Most UNIX system follow this convention: • /home (/people) where the user dir. are at • /usr system info and applications • /usr/local local software (that you install) • /usr/bin /bin standard utilities (like ls, grep) • /usr/etc where SA utilities are located. • /etc system configurations • /dev system devices • /var system logs and other system info • /proc kernel and system info, also images of running processes • /boot (linux only), where the kernel and other files are located for booting.

10. File Types • regular files -rwx------ • directories drwx------ • character device files crwx------ • block device files brwx------ • UNIX domain sockets srwx------ • Named pipes (FIFOs) prwx------ • Symbolic links lrwx------

11. Setting permissions • Use chmod • Can be used in 2 ways • chmod u+x <file> • chmod 700 <file> • Chmod ugo + rx <file> or chmod a + rx <file> u = user, g=group o= world • and chmod ugo – w <file> or chmod a – w <file> • numeric 0 ---, 1 --x , 2 -w-, 3 -wx, 4 r--, 5 r-x, 6 rw-, 7 rwx • so, chmod 744 <file> is -rwxr--r--

12. Extra bits • There is 1 more field for setuid, setgid, and the “sticky bit” • chmod 4755 <file>, enable setuid • The file runs as the owner, instead of who is actually running it. • chmod 2755 <file>, enable setgid • The file runs as the group • chmod 1755 <file>, enable stick bit • It used leave the program in memory, but is historical and linux silently ignores it.

13. Directory Permissions • For directories, the permission are • r: view the contents of the directory, ls • x: you can enter the directory, cd • w: you can create a file in the directory • without it, in linux you can not modify a file either. • SUN, sgi, etc… implement permissions slightly differently. • you don't need write permissions on the directory to modify a file in the directory.

14. Links • Soft links • the “file” is a redirect to where the “real” file is • All file and directory permissions apply. • Can be used across physical hard drives • deleting the link doesn’t delete the file. The “real” file can be deleted and the link will not work anymore. • Hard links • the “file” is a connected directly to the other file. Only the file permissions of the linked file apply. • Can not be used across physical hard drives. • If you deleted the “real” file, the contents still exist for the linked file or vise versa

15. Links (2) • directories can only be soft linked • Using hard links, file locking does not work • ln –s <target file> <linked filename> • creates a soft link • ln <target file> <linked filename> • creates a hard link

16. Note on ext3 size limits • The size of a file and filesystem are dependent on block size • Block is the smallest allocation that can be made in a file.

17. Extra attributes • For the ext2 and ext3 fs • Use lsattr to view, chattr to set/unset • Anyone can set • A, Never update access time • d, no backup (dump will ignore the file) • S, write with no buffering (sort of works) • Causes more problems then is worth. • Only root can set • a, only allow write in append mode • i, make file immutable and undeletable • listed, but don’t work • c, keep contents compressed (transparently) • s, physically erase (write over) when deleted • u, save contents on deletion to permit recovery

18. Default permissions • use the umask command • the system default is to create all files with -rw-r--r-- • directories default to drwxr-xr-x • umask lists the default permissions in reverse • so chmod 755 (rwxr-xr-x), umask 022 • For better security you may want to use a • umask 137 (-rw-r----) • umask 177 (-rw-------) • command: set umask 137

19. Ext4 file system • Ext4 (fourth extended filesystem) • Journaling file system like ext3 • Adopted into the kernel 2.6.28 released Dec 2008 • Support volumes up 1 exabyte and files up to 16 Terabytes • Sub directory limit now 64,000 (32,000 limit in ext3)

20. Ext4 file system (2) • Performance improvements • Delayed allocation, improves performance and reduces fragmentation • Block allocation decisions based on actual file size. • Disadvantage • Delay allocation may lead to data lost in a crash, because old file is already removed, but new data is not written yet. • Ext3 either the old file or new file exists.

21. Quotas • Linux only (others implement differently) • NOTE: quotas and NFS don't work well together. Locally it will tell the user they have exceeded quota, via NFS it may silently discard the file. • add command usrguota and grpquota to /etc/fstab file for each partition using quotas # quotacheck –c /home • creates /home/aquota.user # quotacheck –vug /home • builds a table of disk usage • See chapter 11 for startup scripts.

22. Quotas (2) • Creating quota rules • command edquota –u <user> • NOTE: by default invokes the vi editor • uses blocks instead of kilobytes • a block equals 1K • 512000 blocks is 500MB • 7168000 blocks is 700 MB. • See chapter 11 for more information

23. Quotas (3) • Checking quotas # repquota –a • display a listing of all users, with used space, quota limit, over quota, etc. • using du #du – h –c –s /home/ugrad/* • lists each undergrad's directory with amount of space used (-h show K, M, or GB -c show total at end)

24. Logical Volumes • Technique to deploy large logical volumes, instead of several smaller physical volumes • Can span multiple physical hard drives. • Example: • (3) 9.1 GB drives, use 100 for /boot on one drive • 9.1GB x 3 – 100MB gives about 27GB of space to use, say 20 GB /home and 7Gb for / partation. • Later another 9.1 GB added to extend the volume and then (with ext3) used to expand the /home partition (giving about 29GB). • Note: LVM supports up to 2TB with LVM 2 and 8 Exabytes with 64bit.

25. Q A &